Just last weekend, three liberal writers made the case for taking on Mr. Obama in 2012. Michael Lerner, longtime editor of Tikkun magazine, argued in The Washington Post that a primary represented a “real way to save the Obama presidency,” by forcing Mr. Obama to move leftward. Robert Kuttner, co-founder of The American Prospect and one of the party’s most scathing populist voices, issued a similar call on The Huffington Post, suggesting Iowa as the ideal incubator.

On the same site, Clarence B. Jones, a one-time confidant of the Rev. Dr. Martin Luther King Jr., suggested that liberals should break with Mr. Obama now, just as Dr. King and others did with Lyndon B. Johnson in 1968. “It is not easy to consider challenging the first African-American to be elected president of the United States,” Mr. Jones wrote. “But, regrettably, I believe the time has come to do this.”

Meanwhile, in Iowa, a group known as the Progressive Change Campaign Committee, originally founded to aid Democratic Congressional candidates in 2010, has started broadcasting an advertisement that shows Mr. Obama, in 2008, promising to reverse the tax cuts for the most affluent Americans. The group isn’t advocating a primary challenge just yet — but then, the choice of Iowa as a market seems intended to send a pretty clear warning to the White House.

WIthout an obvious candidate that has emerged that would take on such a primary challenge, so this isn't exactly a pressing concern just yet. However, a primary challenge would still stoke grievances with Obama and could play significant role in damaging Obama's election chances even if a primary challenge, say, petered out in New Hampshire.